Described below is a resistive superconducting current-limiter device, whose conductor track is formed by a superconductor in the form of a strip, whose oxidic high-Tc superconductor material is of the AB2Cu3Ox type, with A being at least one rare earth metal including yttrium, and B being at least one alkaline earth metal. A corresponding current-limiter device is disclosed in EP 0 523 374 A1.
Superconducting metal-oxide compounds with high critical temperatures Tc of above 77 K have been known since 1986, which are therefore referred to as high-Tc superconductor materials, or HTS materials, and, in particular, allow a liquid-nitrogen (LN2) cooling technique. Metal-oxide compounds such as these include in particular cuprates based on specific substance systems, for example of the AB2Cu3Ox type, with A being at least one rare earth metal including yttrium, and B being at least one alkaline earth metal. The main representative of this substance system of the so-called 1-2-3-HTS type is so-called YBCO (Y1Ba2Cu3Ox where 6.5≦x≦7).
The aim is to deposit this known HTS material on different substrates for different purposes, in which case the general aim is to achieve a superconductor material with as high a phase purity as possible. In particular, metallic substrates are therefore provided for conductor applications (see, for example, EP 0 292 959 A1).
With an appropriate conductor structure, the HTS material is in general not deposited directly on a mount strip which is used as a substrate; instead, this substrate strip is first of all covered with at least one thin intermediate layer, which is also referred to as a buffer layer. This buffer layer has a thickness in the order of magnitude of 1 μm and is intended on the one hand to prevent the diffusion of metal atoms from the substrate into the HTS material, which metal atoms could make the superconducting characteristics poorer. On the other hand, the buffer layer is intended to allow a textured structure of the HTS material. Appropriate buffer layers are in general composed of oxides of metals such as zirconium, cerium, yttrium, aluminum, strontium and magnesium, or mixed crystals having a plurality of these metals, and are thus electrically insulating. In a corresponding electrically conductive conductor track, a problem results as soon as the superconducting material changes to the normally conductive state (so-called “quenching”). During this process, the superconductor first of all becomes resistive in places, and thus assumes a resistance R, for example by being heated above the critical temperature Tc of its superconductor material (at so-called “hot spots” or in partial quenching areas), and is in general heated further, so that the layer can burn through.
As a result of this problem, it is known for an additional metallic covering layer composed of an electrically highly conductive material that is compatible with the HTS material, such as silver or gold, to be applied as a shunt, to prevent burning through, directly on the HTS line layer. The HTS material thus makes an electrically conductive contact over an area with the metallic covering layer (see DE 44 34 819 C).
A different type of superconductor in the form of a strip is used for the current-limiter device disclosed in the initially cited EP-A1 document. In this case, the conductor track is manufactured from a superconducting plate with defined dimensions by incorporating side slots so as to produce a meandering shape. Since no normally conductive covering layer is provided in this structure, this results, as before, in a risk of burning through in the area of hot spots.
The hot spots or partial quenching areas which also occur with shunts result in the voltage being distributed non-uniformly along the superconductor layer. In contrast, the voltage U which is applied to the ends is dropped uniformly over the entire length in the substrate strip to which the superconducting layer is applied, and is at an undefined intermediate potential, if the ends are isolated from the applied voltage. In some circumstances, this can result in voltage differences from the conductor track over the buffer layer to the substrate. Because this layer is not very thick, this necessarily leads to electrical flashovers and thus to the buffer layer being destroyed at some points, possibly as well as the superconducting layer. Voltages in the order of magnitude of 20 to 100 volts are typically sufficient for a flashover with buffer layer thicknesses of 1 μm. A corresponding problem occurs in particular when the aim is to produce resistive current-limiter devices using corresponding conductor strips. This is because, in a device such as this, the transition from the superconducting state to the normally conductive state is used for current limiting in the event of a short circuit. It is not possible without problems to make the buffer layer sufficiently voltage-resistant for the normal operating voltages for devices such as these, in the kV range.